Vibration detection vault alarm system



Sept. 1, 1964 P. LAAKMANN VIBRATION DETECTION VAULT ALARM SYSTEM 4Sheets-Sheet l Filed Sept. 7, 1961 Sept. 1, 1964 P. LAAKMANN 3,147,467

VIBRATION DETECTION vAuLT ALARM SYSTEM Sept. 1, 1964 P. I AAKMANNVIBRATION DETECTION VAULT ALARM SYSTEM 4 Sheets-Sheet 3 Filed Sept. 7,1961 4 Sheets-Sheet 4 @aki P. LAAKMANN VIBRATION DETECTION VAULT ALARMSYSTEM Sept. 1, 1964 Filed Sept. 7, 1961 United States Patent O3,147,467 VIBRATION DETECTION VAULT ALARM SYSTEM Peter Laakmann, FortLee, NJ., assignor to American District Telegraph Company, Jersey City,NJ., a corporation of New Jersey Filed Sept. 7, 1961, Ser. No. 136,48714 Claims. (Cl. 340-261) The present invention relates to burglar alarmsystems, and more particularly to burglar alarm systems of the typeespecially adapted for the protec-tion of vaults and similar enclosures.

Burglar alarm systems for vaults and the like operating on the principleof noise detection have been used for many years, and a variety-of suchsystems have been proposed. An early example of such a system is foundin United States Patent 1,192,312, issued July 25, 1916 to R. M. Hopkinsand J. F. D. Hoge. The present invention is concerned primarily withsystems of this general type and in particular with systems responsiveto vibrations produced in attacks on vaults and similar structures.

The construction of vaults varies from essentially monolithic,steel-reinforced, poured concrete structures to masonry structures ofbricks or blocks bonded together with mortar. And an attack on a vaultmay vary from the extreme of a dynamite blast to the scraping away ofmortar with a pointed tool. Modern tools, and especially tools such asportable core drills Wi-th diamond cutters, have been used widely andeffectively by burglars in conducting attacks on vault structures, andsuch tools produce relatively little noise. Sledge hammers and similarnoisy instruments have been used by many burglars in the past to gainentry to some types of vaults, but the more eicient burglars have foundthe hydraulic jack to be a more effective and far less noisy tool forgaining entry to Vaults.

It will be evident that providing an acoustical burglar alarm systemusable with different types of vaults and effective in detecting thedifferent possible modes of attack is a problem of some magnitude.Moreover, a satisfactory system must be operable at a relatively highsensitivity level without an undue occurrence of spurious alarms. Almostany of the systems heretofore used can be made sufficiently sensitive todetect the different modes of attack, but such high sensitivity hasproduced an intolerable false alarm rate. Thus, it has been the practiceto compromise between sensitivity on the one hand and stability andfreedom from false alarms on the other hand.

Ambient noise has been a serious problem in acoustical burglar alarmsystems, and the problem of ambient noise has been greatly aggravated bynoise producing machinery, trucks, and especially jet aircraft. In manycases, and especially in the weaker, more easily attacked vaults, theaverage level of sound of vibratory energy resulting from an attack maybe substantially less than the average ambient sound energy. Forexample, if a vault structure were made of bricks or masonry blocksbonded together by a relatively weak mortar, an attack eifected byscraping away mortar to free one or more blocks might well produce lesssound energy than present as a result of ordinary activities in thesurrounding area.

The principal object of the present invention has been to provide anovel and improved vault alarm system of the acoustical type. While theinvention may be applied with advantage in the protection of the varioustypes of vault structures and under various ambient conditions, theinvention has been found especially useful in the protection of theweaker type of vault structure and particularly where located in an areahaving a high airborne ambient noise level. Typical of such a vaultstrucice ture would be a fur storage vault made from brick or masonryblocks bonded together with mortar.

By weaker type of vault structure is meant those structures which do notpresent a physical barrier of the strength afforded by the monolithicstructures usually used as bank vaults. The term vault is intended torefer to the various types of rooms and compartments used for thestorage of valuables.

An important object of the invention has been the provision of anacoustical type of vault protection system which is effective even whenthe average ambient noise energy level exceeds the average noise energylevel resulting from an attack.

Another object of the invention has been the provision of such a systemwhich is highly responsive to the various possible modes of attack butwhich is resistant to spurious alarms due to ambient noises.

The electrical protection system of the invention detects physicalattacks on a vault or like structure and comprises vibration transducermeans physically disposed relative to the structure to be protected soas to produce a signal voltage proportional to a function of the energyexpended in an attack on the structure to be protected, integratingmeans coupled to the transducer and arranged continuously to average thesignal voltage over a predetermined relative long time interval, andalarm signalling means coupled to the integrating means and arranged toproduce an alarm signal indication when the integrated voltage exceeds apredetermined value.

Other and further objects, features and advantages of the invention willappear more fully from the following description of the invention takenin connection with the appended drawings, in which:

FIG. l is a schematic diagram for use in explaining certain principlesof the invention;

FIG. 2 is another schematic diagram for use in explaining certainprinciples of the invention;

FIG. 3 is a block diagram of a system embodying the invention;

FIGS. 4 and 5, when placed with FIG. 4 above FIG. S, provide a schematicillustration of one form of circuit arrangement embodying the invention;and

FIG. 6 is a schematic diagram illustrating a modification of the circuitof FIG. 5.

Acoustic vault protection systems in the past have been designed toinitiate an alarm signal when the Power P of a noise source, which is afunction of time, reaches some critical value; or, expressed inmathematical terms,

However, in so far as vault protection systems are concerned, there aretwo types of noise. One is the sound produced by normal activity in thevicinity of the protected area, termed ambient noise, and to which thealarm system should not respond. The other is the noise resulting froman actual attack upon the vault, to which the protection system shouldrespond promptly. The problem, therefore, is to distinguish between thetwo types of noise so that false alarms will be avoided while no actualattack is undetected.

Analysis of a burglarious attack upon a vault reveals that the noiseproduced is a byproduct of the mechanical Work expended in the physicalpenetration of the wall. Most of the work is converted to heat by meansof friction and only a small portion thereof is converted to sound.Thus, the sound energy resulting from an -attack may be expressed asN=(KW) (2) where W is the total energy expended and K is an unknownconstant less than unity.

3 Dilferentiating Equation 2 with respect to time yields dN dw -K'a 3)Because N represents the total sound energy, dN/dt represents the energyper unit time or instantaneous power P(t).

Thus

dw P(t) K- (4) where dw/ dt represents the rate of Work expended by theintruder. As stated above, prior protection systems required the valueof dw K to reach a critical level before 1an alarm could be initiated.Thus, through the use of a relatively slow-working tool, such as adiamond ltipped drill (slow as compared to the rate of energy expendedin a sledge hammer attack), or by scraping away mortar, an intrudercould avoid initiating an alarm by keeping his rate of work below thecritical value.

However, while the intruder may vary the rate of work widely, the totalenergy expended, while somewhat dependent upon the type of toolemployed, will not vary nearly as widely. Therefore, the basic energyrelationships can be solved to determine the total work W by combiningEquations 4 and 1 as follows:

K-=F(t)=P(t) (5) and, transposing,

dw=Po dr (6) then integrating both sides l t2 W-KL Pom Therefore, if theprotection system is to successfully discriminate between the soundsproduced -by the mechanical work of an actual attack on the vault andmere ambient noises in the vicinity of the vault, the circuit must be sodesigned as to be responsive to a predetermined value of the integral(7) rather than the simple function P(t). Hence, the output M of thedetecting device expressed as a function of time should be where K is aconstant to be determined for the particular device.

But when the time limits t1 and t2 are taken as zero and infinity, evena small amount of ambient noise would eventually build the integral upto the alarm level. Therefore, the output of the detector must bemodified by the inclusion of a term to counteract its tendency toincrease continuously. Therefore, some portion of detector output whichis represented by QM (i) (9) (wherein Q is a constant less than unity)is continually deducted from the entire detector output, thus (8)becomes carded but the sound energy of a bona tide attack willaccumulate to initiate an alarm when the predetermined level is reached.

The foregoing mathematics may be translated into electrical circuits bymeans of the analog shown in FIG. 1 wherein the noise function P(t) isimpressed on an electrical network 20 in the form of a current i and thedetector output has its analog in the voltage e.

The form of Equation 10 suggests an RC circuit and it has been foundthat the network of FIG. 2 has a solution similar to Equation 10. InFIG. 2, the network 20 of FIG. 1 is Iformed as the parallel combinationof a resistor 21 and a capacitor 22, whose values are R and C,respectively.

Thus in FIG. 2:

Comparison of Equations 14 and l0 shows both equations to be of the sameform and that the constant Q is resembled by the conductance l/R and theconstant K by the capacitance C.

Consequently, the operation expressed by Equation l0 can be performed byan electrical circuit and the system of the invention has been designedto operate on this principle coupled with certain other features to bedescribed below. However, the output of a mechanicalelectrical vibrationtransducer is a function of the vibration intensity and not a functionof the instantaneous Vibration power. And the instantaneous noise poweris proportional to the square of the noise intensity. Thus, in order totranslate rigorously the theoretical concept of Equation 14 into apractical design, an electrical squaring circuit would be necessary. Ithas been found that such a circuit does not provide, significantly moreinformation than that which is alreadyfp'resent in the output of thevibration transducers. Also, if the signal is squared, the output rangeof signals would be even larger than the range of input amplitudes. InView of the difficulties associated with amplifying a large range ofinput amplitudes, a squaring circuit is not illustrated in FIGS. 4

and 5 but could be used if considered desirable.

The system of the invention is illustrated in block diagram form in FIG.3. The 'system input is derived from the interior surface of theAv it tobe protected by means of vibration transducers 2'l vibrationtransducers, a number of which are preferably connected in parallel, aremounted in direct contact with the internal surface of the vault,preferably in an equally spaced pattern. Typically, twelve transducersmight be used, although the number may be varied to suit particularrequirements.

The vibration transducers are :preferably microphones of thepiezoelectric type and my""each comprise direct actuated Rochelle saltcrystal elements arranged in Bimorph construction and housed in adie-cast metallic casing. To ensure reliable sensitivity over largeambient temperature and humidity ranges, the vibration transducer casemay be sealed in a suitable potting compound such as an epoxy resin witha hardening agent so that vibratory energy will be transmitted to thecase and from the case to the crystal elements within the case.Enclosing the vibration detecting elements also serves to shield theseelements from airborne sound except as the latter may produce vibrationsin the vault structure.

Vibration transducers other than those of a crystal type may be used,although the crystal type is preferred. Contact microphones of the typeused on electrical guitars, banjos or other stringed instruments withsounding boards are in general suitable for use in the system of theinvention.

When any one or more f the vibration transducers are subjected to anacceleration, which will occurVwhen the surface to which the transduceris attached vibrates, an alternating voltage will be produced. Thisalternating voltage is applied to a variable attenuator 24 which may beset to adjust the system for the ambient noise level prevalent at aparticular installation and to provide a predetermined systemsensitivity level.

As mentioned above, one of the objects of the invention has been toprovide successful operation when the ambient sound energy is of ahigher average intensity than that of the attack level. This object hasbeen achieved through frequency discrimination obtained by the naturalcharacteristics of the vibration transducers selected and a high-passfilter.

It has been found that the ambient sound energy in vault structures isconcentrated at frequencies below 1000 cycles per second whereas themaximum sound energy produced by an attack thereon lies between 1000 and2000 c.p.s. A high signal to noise ratio is, of course, desirable andthe ratio is dependent upon the frequencies of the sounds involved. Theminimum signal to noise ratio generally is in the frequency range of 50to 1000 c.p.s. because most of the ambient noise energy is concentratedin this region, whereas there is only a small portion of the attackenergy present in this range.

As the frequencies increase, the signal to noise ratio improvesconstantly until it reaches a maximum in the region between 3 and 10kilocycles, even though the attack energy in this region is relativelyvery small. The transducers and subsequent circuitry in the system ofthe invention provide peak performance in this region. Each of thetransducers has an electrical output proportional to the magnitude ofthe acceleration to which it is subjected. Acceleration is measured interms of the gravitation unit G(=981 cm./sec.2), and it has been foundthat a typical scraping attack on a brick wall produces a vibrationtherein of l0-4G between 3 and l0 kc., while the maximum vibrationsproduced by the same attack, regardless of frequency, are lin the orderof -3G. Ambient wall vibrations usually vary between 10'2G and 105G,depending on frequency. Thus, it is evident that attack vibrations liewithin the region of the ambient disturbances. However, in the system ofthe invention successful detection of an attack is accomplished throughfrequency discrimination to permit utilization of the maximum signal tonoise ratio.

Thus, as shown by the block diagram of FIG. 3, the alternating output ofthe transducers is supplied to variable attenuator 24 for systemsensitivity adjustment, then to a high-pass filter 25 which cuts off allfrequencies below 3 kc. existing in the output of the transducers. Thefiltered transducer output signal is supplied to a low noise A.C.amplifier 26 having a flat response between 3 and 10 kc. This bandwidthminimizes the noise caused by thermal agitation of the resistors,transistors and like components. The amplifier generally will operatewith an input signal only a few db above the inherent noise of theamplifier.

The amplifier output is rectified by a full wave rectifier Z7, producinga pulsating current which is a function of the instantaneous wallvibrations in the frequency region from 3 to 10 kc. The rectifier outputis fed to an integrating network 28 which averages the rectifier outputover a suitable time interval, preferably about 15-25 minutes. Thus theoutput of the integrating network is proportional to the average wallvibrations over a selected period. It should be understood that thelonger the time interval over which averaging occurs the less likely itwill be that an attack will be undetected, since evenan extremely slowattack will eventually produce a substantial integrated output. Whilefor practical purposes an integrating time interval of from about 15 to25 minutes is preferred, an integrating time interval as low as one-halfminute will provide improved results over systems previously used, whilean integrating time interval of more than 25 minutes and as much asseveral hours might be used to advantage in special cases. Such specialcases would be ones where a very slow rate of work in an attack waspossible.

The integrating network 28 is followed by a differentiating network 29having a time constant which is long relative to the integrating networktime constant. For example, the time constant of the differentiatingnetwork might be about 30 to 60 minutes. The network 29 may be omittedif a sufficiently high signal to noise ratio has already been achieved.The differentiating network 29 will pass all signals varying at a fasterrate than its time constant and differentiates only those signals whichexist over a long period of time. The differentiating network acts tocancel the effect of amplifier noise and does so because thecontribution of such noise to the integrated output is a constant whichcan be eliminated by subsequent differentiation. Differentiation alsoserves to prevent ambient noises which vary over a long period of timefrom causing a false alarm, since such slow varying signals will not bepassed.

The output of differentiating network 29 is supplied to a voltagesensitive detector 30 which actuates an alarm indicating device when thevoltage reaches a predetermined level. The alarm indicating device ishere suggested by conductors 31 and normally open relay contacts 32;when detector 30 is actuated, contacts 32 close, completing whateveralarm circuit may be connected to conductors 31. It is highly desirablethat the alarm signal be transmitted to a central station, guard stationor police headquarters, but a local audible or visual alarm may beprovided as is well known in the art.

If the differentiating circuit is used, a slow varying ambient noise,while insufficient to produce an alarm, may eventually reach asufficient amplitude to overdrive the amplifier with a consequentdecrease in amplifier gain and system sensitivity. Such a situationmight prevent detection of an attack. To avoid this possibility, thedifferentiating network is bypassed, as shown by the block 33, thebypass being effective when the output of the integrating networkexceeds a predetermined value. This bypass may conveniently be effectedby connecting a Zener diode across the differentiating network andselecting the diode to become conductive at the desired value of theintegrating network output voltage.

There is a possibility that the short duration but high intensity of adynamite blast or a similar impact could overdrive the amplifier soheavily that the contribution to the integrated output would beinsufficient toproduce an alarm. To overcome this possibility an impactchannel has been provided. As shown mathematically above, theintegrating operation provided in the integrating channel will provideapproximately equal sensitivity for different kinds of attacks eventhough they vary widely in instantaneous amplitudes and time duration.The mathematics, however, presume a completely linear system. Butcompletely linear accelerometers, amplifiers and rectifiers cannot beconstructed to operate over the wide range of amplitudes that isrequired. As an example, suppose that the lowest wall vibrations whichare being detected are of the order of 104G and that these vibrationsproduce an output voltage at the amplifier which is below the maximumpossible output of the amplifier by a factor of 10. If a dynamite blastnow occurs, the wall vibrations will easily reach amplitudes of 10G ormore resulting in a l change in vibrations and 1010 change in power.Obviously, no amplifier could handle such a variation in input signalamplitudes and its output will be considerably less than that of thetheoretical completely linear amplifier. Consequently, the integratedsignal will be less and not directly related to wall vibrations as isindicated by the mathematical analysis.

To avoid a defeat of the system by such attack, an impact channel hasbeen added in parallel with the integrating channel. The impact channelcomprises a variable attenuator 34 for system sensitivity adjustment, alow gain amplifier 35 operating in a frequency range such as 500 to10,000 c.p.s. and a rectifier 36 which feeds signals directly to thevoltage sensitive detector 30.

The voltage sensitive detector functions upon receipt of a signal ofsufficient voltage from either the integrating channel or the impactchannel to operate the usual alarm indicating devices at the centralstation or other remote place and/or local alarm devices.

Referring now to FIGS. 4 and 5, the vibration transducers 23 areconnected in parallel across the winding of a potentiometer 37 andacross the winding of a potentiometer 38. The potentiometer 37 serves asthe variable attenuator for the impact channel while the potentiometer38 serves as the variable attenuator for the integrating channel.

The slider and one end of potentiometer 38 are connected to respectiveinput terminals of a high-pass filter formed by capacitors 39, 40 and 41and coils 42 and 43. The cutoff frequency of the high-pass filter isselected so as to pass the frequencies predominant in vibrationsresulting from an attack and to suppress the frequencies predominant inambient noise. As suggested above, this cutoff frequency will be of theorder of 3 k.c. in most installations. The filter impedance ispreferably selected as equal to the average impedance of the parallelconnected vibration transducers, e.g., 3000 ohms.

The output terminals of the high-pass filter are connected to therespective sides of the primary winding of an input transformer 44 byconductors 45 and 46. The secondary winding of transformer 44 is coupledto the input circuit of a six stage high gain amplifier comprisingdirect coupled transistors 47, 48, 49, 50, 51 and 52. The transistors47-52 might be, for example, of the 2N336 type.

The high gain amplifier input voltage from transformer 44 is supplied toa resistor 53 through a coupling capacitor 54. One end of resistor 53 isconnected to the base of transistor 47 and the other end thereof iscoupled to a negative supply conductor 55 through a capacitor 56. Thecapacitor 56 has a high capacitance, so as to effectively connectresistor 53 and conductor 55 at signal frequencies.

Negative supply conductor 55 and corresponding positive supply conductor57 are coupled to voltage supply input terminals 58 and 59 through arectifier circuit 60 which comprises rectifiers 61, 62, 63 and 64arranged to maintain the polarities of conductors 55 and 57 irrespectiveof the supply voltage polarity. A diode 65, preferably of the Zenertype, is connected between conductors 55 and 57 for protection in theevent of voltage surges in the supply and a capacitor 66 is connectedbetween conductors 55 and 57 to smooth the supply voltage.

The collector of each of transistors 47-52 is coupled to positive supplyconductor 57 through a respective one of resistors 67, 68, 69, 70, 71and 72. The collector of each of transistors 47-51 is also directlyconnected to the base of the following transistor. The emitter of eachof transistors 47-51 is Connected to negative supply conductor 55 andthe emitter of transistor 52 is coupled to conductor 55 through aresistor 73.

The amplifier output appears at the collector of transistor 52.

Stability of the transistor amplifier operating point is achieved bymeans of a first feedback connection, including a transistor 74, whichmight be of the 522A type. The base of transistor 74 is coupled to thecollector of transistor 52 through a resistor 75, the emitter thereof isconnected to the junction of resistors 76 and 77 forming a voltagedivider between conductors 55 and 57, and the collector thereof iscoupled to the base of input transistor 47 through resistor 53. Acapacitor 78 is connected between the base of transistor 74 and negativeconductor 55.

If the D.C. operating voltage at the collector of transistor 52 shoulddeviate from -its normal value, e.g., seven volts, the resulting changein voltage at the base of transistor 74 causes a change in the collectorcurrent of transistor 74. Since part of the collector output oftransistor 74 supplies the base current for transistor 47, a change inthe collector current of transistor 74 will change the operating pointof transistor 47. The feedback loop is negative, so that a change inamplifier operating point will cause a change in the collector currentof transistor 74 in a sense to return the amplifier operating point toits normal value. The corrective feedback loop has a large time constantcompared to the operating frequencies so that signal voltages do notexperience corrective feedback through this loop.

The amplifier gain is stabilized by a second feedback arrangement formedby resistors 79, 80, 81, 82 and 83, each of which is connected betweenthe collector and base of a respective one of transistors 47-51. Theresistors 79-83 form individual feedback loops for the individualstages, which overcomes the tendency to regenerate at the higherfrequencies which would exist were one overall feedback loop used With asix stage amplifier. Direct connection of the feedback resistors betweencollector and base is possible because the collector voltage of eachstage is equal to the base to emitter voltage so that there will only bea very small cornonent of D.C. current fiow from the collector to thease.

In order to keep the amplifier noise output voltage as low as possible,the amplifier band width should be designed to exclude so far aspossible the major ambient noise frequency components. For example, theamplifier. from the base of transistor 47 to the collector of transistor52 might have a band width of three kilocycles with a center frequencyof 4.5 kc. The amplifier lower frequency cutoff is determined by a thirdnegative feedback arrangement afforded by a feedback loop between thebase of transistor 52 and the collector of transistor 48. This feedbackloop comprises resistors 84 and 85 connected in series between the baseof transistor 52 and the collector of transistor 48 and is frequencysensitive because of the low frequency suppression afforded by acapacitor 86 coupled between the junction of resistors 84 and 85 andnegative supply conductor 55. The capacitor 86 tends to shunt highfrequency components out of the negative feedback loop thus affordingless negative feedback at the higher frequencies. High frequency cutoffis determined by a capacitor 87 coupled between the collector oftransistor 49 and positive supply conductor 57.

The high gain amplifier output from the collector of transistor 52 issupplied through a capacitor 88 and a conductor 89 to a rectifiercircuit formed by rectifiers 90 and 91 connected as a voltage doublercircuit.

The rectified voltage output of the voltage doubler appears acrossparallel connected capacitor 92 and resistor 93, which form theintegrating circuit. Typically, resistor 93 might have a value of 5megohms and capacitor 92 might have a value of 250 microfarads, yieldinga time constant of about 2O minutes. The voltage across capacitor 92 andresistor 93 will be proportional to the integral function as discussedabove in connection with Equations and 14. y

A differentiating network formed by a capacitor 94 and a resistor 95 iscoupled across the integrating circuit. The differentiated signal outputappearing across resistor 95 is supplied through a coupling diode 96 toone end of the coil of a relay 97. The differentiating circuit has asubstantially higher time constant than the integrating circuit,preferably more than one and one-half times as high. For example, thecapacitor 94 might have a value of 250 microfarads and the resistor 95 avalue of megohms, yielding a time constant of about 83 minutes.

The integrating circuit produces an output signal in the form of apulsating direct voltage whose value represents the average detectedvibration over a period determined by the integrating circuit timeconstant. The differentiating network will pass all signals which Varyat a faster rate than its time constant. Slow varying signals will,however, be suppressed.

A diode 98, which is preferably of the Zener type, is shunted acrosscapacitor 94 and resistor 95, i.e., across the differentiating circuit.Normally the Zener diode 98 does not contribute to the network output.However, if the integrating network output, i.e., the voltage acrosscapacitor 92 and resistor 93, exceeds the Zener diode breakdown voltage,eg., 7 volts, the Zener diode will break down. With the Zener diode 98conductive the differentiating network is bypassed and a contribution ismade to the network output equal to the Zener diode breakdown voltage.

This bypassing of the differentiating network is provided to overcomethe possibility that an ambient noise may reach such a large amplitudeas to overdrive the amplifier, with consequent decrease in amplifiergain and system sensitivity, and yet vary so slowly as to be suppressedby the differentiating circuit. With the bypass afforded by the Zenerdiode 98, the signal output required for alarm registration is reducedby the amount of the Zener breakdown voltage. When the voltage acrossthe integrating network drops below the Zener voltage, the Zener diode98 will become non-conducting and the differentiating network will againbe effective.

As mentioned above, the diiferentiating network output voltage issupplied to one end of the coil of relay 97. The other end of the coilof relay 97 is connected to the emitter of a unijunction transistor ordouble base diode 99, which might be of the 2N498 type. Base B2 ofunijunction transistor 99 is connected to the positive supply conductor57 and base B1 thereof is connected to negative supply conductor 55. Thereturn path for the integrating channel output signal supplied to theemitter of unijunction transistor 99 through the coil of relay 97 iscompleted through a resistor 100 and a potentiometer 101, the slider ofthe latter being connected to the junction of rectifier 90, capacitor 92and resistors 93 and 95.

Unijunction transistor 99 will not conduct until the voltage between itsemitter and base B1 equals or exceeds a predetermined percentage of thevoltage between bases B1 and B2 thereof, e.g., 60%. In other words, theunijunction transistor will not conduct until a predetermined signalvoltage is supplied to the emitter of transistor 99 from thedifferentiating network or from the integrating network in combinationwith Zener diode 98. Assuming a 14 Volt supply potential, the B1-B2voltage will be 14 volts and the signal voltage required to renderunijunction transistor 99 conductive would be about 8.4 volts. The alarmsignal voltage is that existing between the junction of capacitor 94 andresistor 95 and negative supply conductor 55, or, in other'words, thesum of the voltage across resistors 95 and 119 and a portion ofpotentiometer 101.

When unijunction transistor 99 becomes conductive,

an energizing circuit for relay 97 is momentarily completed and therelay 97 picks up. The circuit is energized by the charge on capacitor120. The current flows through diodes 102, 103, relay 97, the emitterand base B1 of transistor 99, and back to conductor 55. A capacitor 120connected between one side of the coil of relay 97 and conductor 55provides a triggering action which assists in the initial discharge ofcapacitor 120. Energization of relay 97 results in closing of contact104 thereof, completing the central station or other alarm signalcircuit and transmitting an alarm signal.

Unijunction transistor 99 may also be energized by the impact channel.The impact channel derives its input from a transformer the primarywinding of which is connected to potentiometer 37. One end of thesecondary winding of Ytransformer 105 is connected to the base of atransistor 106 and the other end thereof is connected to the junction ofresistors 107 and 108 connected as a voltage divider between supplyconductors 55 and 57. This end of the secondary winding of transformer105 is also coupled to the emitter of transistor 106 through arelatively large capacitor 109. The emitter circuit of transistor 106 iscompleted to negative supply conductor 55 through a resistor 110.

The collector of transistor 106 is coupled to positive supply conductor57 through a resistor 111 and is coupled to the base of a transistor 112through a coupling capacitor 113. A resistor 114 is connected from thebase of transistor 112 to the junction of resistors 107 and 108 toprovide bias for transistor 112. The collector of transistor 112 iscoupled to positive supply conductor 57 through a resistor 115. Theemitter of transistor 112 is coupled to negative supply conductor 55through the parallel combination of a resistor 116 and a capacitor 117.

Transistors 106 and 112 form a low gain broad band amplifier which willamplify the wide range of frequencies associated with heavy impacts suchas those of a dynamite or other blast. The amplified output is suppliedfrom the collector of transistor 112 through a capacitor 118 to thejunction of rectiers 102 and 103 which are connected as a voltagedoubler. The voltage doubler circuit is completed by a resistor 119 anda capacitor 120 connected in parallel between one end of rectifier 102and negative supply conductor 55.

The rectified output of the voltage doubler 102-103 is supplied throughthe coil of relay 97 to the emitter of unijunction transistor 99 andwill cause the latter to conduct if this rectified output issufficiently great, i.e., if this rectified output is sufficient to makethe emitter-base B1 voltage of the unijunction transistor at least apredetermined percentage of the base Bl-base B2 voltage. Energization ofunijunction transistor 99 results in momentary energization of relay 97,as described above, and hence results in transmission of an alarmsignal.

The sensitivity of unijunction transistor 99 can be adjusted by changingthe position of the slider of the potentiometer 101. This slider isconnected to the junction of capacitors 121 and 122 connected in seriesbetween conductors 55 and 57.

The unijunction transistor 99 may conveniently be replaced with a fieldeffect device such as a field effect transistor or a field effecttetrode. Such a field effect device affords the reliability and otheradvantages of a solid state device but permits a continuous output whichis desirable for recording ambient sound level in a new installation.

A modification of a part of the circuit of FIG. 5 to use a field effecttransistor is shown in FIG. 6. The field effect transistor is shown at123 and might be of the C4612 type. The grid of field `effect transistor123 is connected to the junction of capacitor and rectifier 103 and thecathode thereof is coupled to negative supply conductor 55 through avariable resistor 124. The anode of transistor 123 is coupled topositive supply conductor 57 through the coil of an alarm relay 125 anda jack 126.

A resistor 127 is coupled between the cathode and anode of transistor123.

Relay 125 is arranged to become energized when the anode current oftransistor 123 reaches a value corresponding to an input voltage atwhich unijunction transistor 99 of FIG. 5 become conductive.Energization of relay 125 closes contacts 128 thereof, transmitting anYalarm signal as in the case of contacts 104.

By connecting a recorder to jack 126, a record may be made of thetransistor 123 output. This record will be proportional to theintegrating and impact channel outputs and hence to the ambient noiseand provides use- Aful information in setting the channel sensitivities.

While the invention has been described in connection with a specificembodiment thereof and in a specific use, various modifications thereofwill occur to those skilled in the art Without departing from the spiritand scope of the invention as set forth in the appended claims.

What is claimed is:

l. An electrical protection system for detecting physical attacks on avault or like structure, comprising vibration transducer means disposedin close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and arranged to produce an alternatingsignal voltage proportional to the magnitude of the acceleration towhich it is subjected, means to filter said signal voltage to suppressfrequency components therein below a predetermined value, a firstamplifier, means to Vsupply the filtered signal voltage output of saidfilter means to the input circuit of said first amplifier, a rectifyingcircuit coupled to the output of said first amplifier and arranged toproduce a pulsating direct current from the amplified signal voltageoutput of said first amplifier, an integrating network having arelatively long time constant and coupled to the output of saidrectifier circuit to integrate said pulsating direct current, adifferentiating network having a time constant substantially longer thansaid relatively long time constant and coupled to the output of saidintegrating network to differentiate the integrated pulsating directcurrent output of said integrating circuit, a detector arranged toproduce an alarm signal indication when the input supplied theretoexceeds a predetermined value, means to derive an output from saiddifferentiating network and to apply the same as an input to saiddetector, bypass means coupled to said integrating and differentiatingnetworks and arranged to bypass said dierentiating network when theoutput of said integrating network exceeds a predetermined value therebyto apply said output of said integrating network to said detector as aninput potential, a second amplifier, means to apply said signal voltageto the input of said second amplifier, means to rectify the output ofsaid second amplifier, and means to apply the rectified output of saidsecond amplifier to said detector as an input potential.

2. An electrical protection system for detecting physical attacks on avault or like structure, comprising vibration transducer means disposedin close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and arranged to produce an alternatingsignal voltage proportional to the magnitude of the acceleration towhich it is subjected, means to filter said signal voltage to suppressfrequency components therein below about 3000 cycles per second, a firstamplifier, means to supply the filtered signal voltage output of saidfilter means to the input circuit of said first amplifier, a rectifyingcircuit coupled to the output of said first amplifier and arranged toproduce a pulsating direct current from the amplified signal voltageoutput of said first amplifier, an integrating network having arelatively long time constant and coupled to the output of saidrectifier circuit to integrate said pulsating direct current, adifferentiating network having a time constant substantially longer thansaid relatively long time constant and coupled to the output of saidintegrating network to differentiate the integrated pulsating directcurrent output of said integrating circuit, a voltage-sensitive detectorarranged to produce an alarm signal indication when the input potentialsupplied thereto exceeds a predetermined value, means to derive avoltage from said differentiating network and to apply the same as aninput potential to said detector, a second amplifier, means to applysaid signal voltage to the input of said second amplifier, means torectify the output of said second amplifier, and means to apply therectified output of said second amplifier to said detector as an inputpotential.

3. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and each arranged to produce analternating output voltage proportional to the magnitude of theacceleration to which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said walls, means to filter said signal voltage tosuppress frequency components therein below about 3000 cycles persecond, a first plural stage amplifier having a substantially fiatresponse over a predetermined frequency range having a lower limit ofabout 3000 cycles per second, means to supply the filtered signalvoltage output of said filter means to the input circuit of said firstamplifier, a rectifying circuit coupled to the output of said firstamplifier and arranged to produce a pulsating direct current from theamplified signal voltage output of said first amplifier, an integratingnetwork having a time constant lying in the range of about l5 to 25minutes and coupled to the output of said rectifier circuit to integratesaid pulsating direct current, a voltage-sensitive detector arranged toproduce an alarm signal indication When the input potential suppliedthereto exceeds a predetermined value, means to apply the output voltageof said integrating network to said detector as an input potential, asecond amplifier having a gain substantially less than the gain of saidfirst amplifier, means to apply said signal voltage to the input of saidsecond amplifier, means to rectify the output of said second amplifier,and means to apply the rectified output of said second amplifier to saiddetector as an input potential.

4. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure Walls and each arranged to produce analternating output voltage proportional to the magnitude of theaccelera-tion to which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said Walls, means to filter said signal voltage tosuppress frequency components therein lbelow about 3000 cycles persecond, a plural stage amplifier having a substantially flat responseover a predetermined frequency range having a lower limit of about 3000cycles per second, means to supply the filtered signal voltage output ofsaid filter means to the input circuit of said amplifier, a rectifyingcircuit coupled to the output of said amplifier and arranged to producea pulsating direct current from the amplified signal voltage output ofsaid amplifier, an integrating network having a time constant lying inthe range of about l5 to 25 minutes and coupled to the output of saidrectifier circuit to integrate said pulsating direct current, adifferentiating network having a time constant lying in the range of'about 30 to 60 minutes and coupled to the output of said integratingnetwork to differentiate the integrated pulsating direct current outputof said integratingcircuit, a voltage-sensitive detector arranged toproduce an alarm signal indication when the input po- Air tentialsupplied thereto exceeds a predetermined value, means to derive avoltage from said differentiating network and to apply the same as aninput potential to said detector, and bypass means coupled to saidintegrating and differentiating networks and arranged to bypass thelatter when the output voltage of said integrating network exceeds apredetermined value thereby to apply said output voltage of saidintegrating network to said detector as an input potential.

5. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and each arranged to produce analternating output voltage proportional to the magnitude of theacceleration lto which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said walls, means to filter said signal voltage tosuppress frequency components therein below about 3000 cyclesper second,a first plural stage amplifier having a substantially flat response overa predetermined frequency range having a lower limit of about 3000cycles per second, means to 4supply the lfiltered signal voltage outputof said filter means to the input circuit of said first amplifier, arectifying circuit coupled to the output of said first amplifier andarranged to produce a pulsating direct current from the amplified signalvoltage output of said first amplifier, an integrating network having atime constant lying in the range ofabout 15 to 25 minutes and coupled tothe o-utput of said rectifier circuit to integrateV said pulsatingdirect current, a differentiating network having a time constant lyingin the range of about 30 to 60 minutes and coupled to the output of saidintegrating network to difierentiate the integrated pulsating directcurrent output of said integrating circuit, a voltage-sensitive detectorarranged to produce an alarm signal indication when the input potentialsupplied thereto exceeds a predetermined value, means to derive avoltage from said differentiating network and to apply the same as aninput potential to said detector, bypass means coupled to saidintegrating and differentiating networks and arranged to bypass thelatter when the output voltage of said integrating network exceeds apredetermined value thereby to apply said output voltage of saidintegrating network to said detector as an input potential, a secondamplifier having a gain substantially less than the gain of said firstamplifier, means to apply said signal Voltage to the input of saidsecond amplifier, means to rectify the output of said second amplifier,and means to apply the rectified output of said second amplifier to saiddetector as an input potential.

6. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and each arranged to produce analternating output voltage proportional to the magnitude of theacceleration to which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said walls, means to filter said signal voltage tosuppress frequency components therein below a predetermined frequency, afirst plural stage amplifier having a substantially flat response over afrequency range lying above said predetermined frequency, afrequency-sensitive negative feedback loop coupled to said firstamplifier and having a frequency response tending to suppressamplification of frequencies lying outside said range, means to supplythe filtered signal voltage output of said filter means to the inputcircuit of said first amplifier, a rectifying circuit coupled to theoutput of said first amplifier 4and arranged to produce a pulsatingdirect current from the amplified signal voltage output of saidamplifier, an

integrating network having a time constant of many minutes and coupledto the output of said rectifier circuit to integrate said pulsatingdirect current, a differentiating network having a time constantsubstantially longer `than the time constant of said integrating networkand coupled to the output of said integrating network to differentiatethe integrated pulsating direct current output of said integratingcircuit, a voltage-sensitive detector arranged to produce an alarmsignal indication when the input potentional supplied thereto exceeds apredetermined value, means to derive a voltage from said differentiatingnetwork and to apply the same as an input potential to said detector,bypass means coupled to said integrating and differentiating networksand arranged to bypass the latter when the output voltage of saidintegrating network exceeds a predetermined value thereby to apply saidoutput voltage of said integrating network to said detector as an inputpotential, a second amplifier having a gain substantially less than thegain of said first amplifier, means to apply said signal voltage to theinput of said second amplifier, means to rectify the output of saidsecond amplifier, and means to apply the rectified output of said secondamplifier to said detector as an input potential.

7. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and each arranged to produce analternating output voltage proportional to the magnitude of theacceleration to which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said walls, means to filter said signal voltage tosuppress frequency components therein below a predetermined frequency, afirst plural stage transistor amplifier having a substantially fiatresponse over a frequency range lying above said predeterminedfrequency, a first negative feedback loop intercoupling the output andinput circuits of said amplifier and having a long time constant at thefrequencies of said signal voltage whereby said first negative feedbackloop stabilizes the operating point of said first amplifier, a secondnegative feedback loop coupled to said first amplifier and havingfrequency-sensitive components selected and arranged to suppressamplification of signal frequencies outside said range, a rectifyingcircuit coupled to the output of said first amplifier and arranged toproduce a pulsating direct current from the amplified signal voltageoutput of said amplifier, an integrating network having a relativelylong time constant and coupled to the output of said rectifier circuitto integrate said pulsating `direct current, a voltage-sensitivedetector arranged to produce an alarm signal indication when the inputpotential supplied thereto exceeds a predetermined value, means to applythe output Voltage of said integrating network to said detector as aninput potential, a second amplifier having a gain substantially lessthan the gain of said first amplifier, means to apply said signalVoltage to the input of said second amplifier, means to rectify theoutput of said second amplifier, and means to apply the rectified outputof said second amplifier to said detector as an input potential.

8. A'n electrical protection system as set forth in claim 7 in whichsaid voltage sensitive detector comprises a field effect device and inwhich said input potentials are applied to the input electrode of saidfield effect device.

9. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and each arranged to produce analternating output voltage proportional to the magnitude of theacceleration to which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said walls, means to filter said signal voltage tosuppress frequency components therein below a predetermined frequency, afirst high gain plural stage amplifier having a substantially fiatresponse over a frequency range lying above said predeterminedfrequency, means to supply the filtered signal voltage output of saidfilter means to the input circuit of said first amplifier, a rectifyingcircuit coupled to the output of said first amplifier and arranged toproduce a pulsating direct current from the amplified signal voltageoutput of said amplifier, an integrating network having a time constantlying in the range of about 15 to 25 minutes and coupled to the ouput ofsaid rectifier circuit to integrate said pulsating direct current, adifferentiating network having a time constant lying in the range ofabout 30 to 60 minutes and coupled to the output of said integratingnetwork to differentiate the integrated pulsating direct current outputof said integrating circuit, a unijunction transistor aranged as avoltage-sensitive detector to produce an alarm signal indication whenthe input potential applied between the emitter and one of the bases ofsaid unijunction transistor exceeds a predetermined value, means toderive a voltage from said differentiating network and to apply the sameas an input potential to said unijunction transistor emitter-basecircuit, bypass means comprising a diode element connected across saiddifferentiating network and arranged to bypass said differentiatingnetwork when the output voltage of said integrating network exceeds apredetermined value thereby to apply said output voltage of saidintegrating network to said unijunction transistor emitter-base circuitas an input potential, a second low gain amplifier, means to apply saidsignal voltage to the input of said second amplifier, means to rectifythe output of said second amplifier, and means to apply the rectifiedoutput of said second arnplifier to said unijunction transistoremitter-base circuit as an input potential.

10. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibratoryenergy in the structure walls and each arranged to produce analternating output voltage proportional to the magnitude of theacceleration to which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said walls, means to filter said signal voltage tosuppress frequency components therein below a predetermined value, afirst amplifier having a substantially fiat response over a frequencyrange lying above said predetermined value, means to supply the filteredsignal voltage output of said filter means to the input circuit of saidfirst amplifier, rectifying means coupled to the output of said firstamplifier to rectify the output thereof, means coupled to saidrectifying means to integrate the output thereof over a relatively longtime interval and to produce an integrated first output potential havinga magnitude proportional to the detected vibration intensity in saidwalls over said relatively long time interval, a second amplifiercircuit having a gain substantially less than the gain of said firstamplifier circuit, means to apply said signal voltage to the input ofsaid second amplifier, means to derive from the output of said secondamplifier circuit a second output potential proportional to theinstantaneous vibration intensity in said walls, signalling meansresponsive to an applied input potential greater than a predeterminedvalue to transmit an alarm signal, and means t supply each of said firstand second output potentials to said signalling means as an inputpotential.

l1. An electrical protection system for detecting physical attacks on avault or like structure, comprising a plurality of vibration transducersdisposed in close proximity to the inside surface of the structure to beprotected so as to be subject to acceleration resulting from vibra- -16tory energy in the structure walls and each arranged to produce analternating output voltage proportional to the magnitude of theacceleration to which it is subjected, means to combine said outputvoltages into a composite signal voltage proportional to the intensityof vibrations in said walls, means to filter said signal voltage tosuppress frequency components therein below about 3000 Vcycles persecond, a first plural stage amplifier having a substantially fiatresponse over a predetermined frequency range having a lower limit ofabout 3000 cycles per second, means to supply the filtered signalvoltage output of said filter means to the input circuit of said firstamplifier, a rectifying circuit coupled to the output of said firstamplifier and arranged to produce a pulsating direct current from theamplified signal voltage output of said first amplifier, an integratingnetwork having a relatively long time constant and coupled to the outputof said rectifier circuit to integrate said pulsating direct current, afield effect transistor, signalling means coupled to the anode of saidfield effect transistor and arranged to produce an alarm signal when theanode current of said field effect transistor exceeds a predeterminedvalue, means to apply the output voltage of said integrating network tothe grid of said field effect transistor, a second amplifier having again substantially less than the gain of said first amplifier, means toapply said signal voltage to the input of said second amplifier, meansto rectify the output of said second amplifier, and means to apply therectified output of said second amplier to the grid of said field effecttransistor.

12. An electrical protection system for detecting physical attacks on avault or like structure, comprising vibration transducer meansphysically disposed relative to the structure to be protected so as toproduce a signal voltage proportional to a function of the energyexpended in an attack on said structure to be protected, rectifyingmeans coupled to said transducer means to rectify said signal voltage,integrating means coupled to said rectifying means and arrangedcontinuously to average said rectified signal voltage over apredetermined relatively long time interval, differentiating meanscoupled to said integrating means and having a time constantsubstantially longer than said relatively long time interval, and alarmsignalling means coupled to said differentiating means and arranged toproduce an alarm signal indication when the integrated voltage exceeds apredetermined value.

13. An electrical protection system for detecting physical attacks on avault or like structure, comprising vibration transducer meansphysically disposed relative to the structure to be protected so as toproduce a signal voltage proportional to a function of the energyexpended in an attack on said structure to be protected, rectifyingmeans coupled to said transducer means to rectify said signal voltage,integrating means coupled to said rectifying means and arrangedcontinuously to average said rectified signal voltage over apredetermined relatively long time interval, said integrating meanscomprising a capacitive element, a charging circuit for said capacitiveelement and a resistive element coupled in parallel with said capacitiveelement, differentiating means coupled to said integrating means andhaving a time constant substantially longer than said relatively longtime interval, and alarm signalling means coupled to saiddifferentiating means and arranged to produce an alarm signal indicationwhen the integrated voltage across said capacitive element exceeds apredetermined value.

14. An electrical protection system for detecting physical attacks on avault or like structure, comprising vibration transducer meansphysically disposed relative to the structure to be protected so as toproduce a signal voltage proportional to a function of the energyexpended in an attack on said structure to be protected, rectifyingmeans coupled to said transducer means to rectify said signal voltage,integrating means coupled to said rectifying means and arrangedcontinuously to average said 17 rectied signal voltage over apredetermined relatively long time interval greater than about one-halfminute and less than about 25 minutes, differentiating means coupled tosaid integrating means and having a time constant lying in the range ofabout 30 to 60 minutes, alarm sig- 5 nalling means coupled to saiddifferentiating means and arranged to produce an alarm signal indicationwhen the integrated voltage exceeds a predetermined value, and meanscoupled to said transducer and to said alarm signalling means andresponsive only to a signal voltage 10 above a selected level to causesaid alarm signalling means to produce an alarm signal indicationindependently of the value of said integrated Voltage.

References Cited in the file of this patent UNITED STATES PATENTS

1. AN ELECTRICAL PROTECTION SYSTEM FOR DETECTING PHYSICAL ATTACKS ON AVAULT OR LIKE STRUCTURE, COMPRISING VIBRATION TRANSDUCER MEANS DISPOSEDIN CLOSE PROXIMITY TO THE INSIDE SURFACE OF THE STRUCTURE TO BEPROTECTED SO AS TO BE SUBJECT TO ACCELERATION RESULTING FROM VIBRATORYENERGY IN THE STRUCTURE WALLS AND ARRANGED TO PRODUCE AN ALTERNATINGSIGNAL VOLTAGE PROPORTIONAL TO THE MAGNITUDE OF THE ACCELERATION TOWHICH IT IS SUBJECTED, MEANS TO FILTER SAID SIGNAL VOLTAGE TO SUPRESSFREQUENCY COMPONENTS THEREIN BELOW A PREDETERMINED VALUE, A FIRSTAMPLIFIER, MEANS TO SUPPLY THE FILTERED SIGNAL VOLTAGE OUTPUT OF SAIDFILTER MEANS TO THE INPUT CIRCUIT OF SAID FIRST AMPLIFIER, A RECTIFYINGCIRCUIT COUPLED TO THE OUTPUT OF SAID FIRST AMPLIFIER AND ARRANGED TOPRODUCE A PULSATING DIRECT CURRENT FROM THE AMPLIFIED SIGNAL VOLTAGEOUTPUT OF SAID FIRST AMPLIFIER, AN INTEGRATING NETWORK HAVING ARELATIVELY LONG TIME CONSTANT AND COUPLED TO THE OUTPUT OF SAIDRECTIFIER CIRCUIT TO INTEGRATE SAID PULSATING DIRECT CURRENT, ADIFFERENTIATING NETWORK HAVING A TIME CONSTANT SUBSTANTIALLY LONGER THANSAID RELATIVELY LONG TIME CONSTANT AND COUPLED TO THE OUTPUT OF SAIDINTERGRATING NETWORK TO DIFFERENTIATE THE INTEGRATED PULSATING DIRECTCURRENT OUTPUT OF SAID INTERGRATING CIRCUIT, A DETECTOR ARRANGED TOPRODUCE AN ALARM SIG-